Due to its complexity, a baby’s digestive system is not yet fully developed at the time of birth and is characterised by limitations with regard to volume and motility. In order to reach full functionality, an adjustment from in utero conditions to the new environment is necessary. Nutrients provided by breast milk facilitate this process.
The microbiome – a community of various bacteria present in the intestine – is particularly important for the development of a robust immune system. As such, a healthy microbiome is essential for avoiding immune diseases such as food allergies and asthma. As the microbiota continues to evolve after birth, breast milk and weaning greatly influence this process.
The enzymatic activity of the intestine is a critical prerequisite for proper digestion. Enzymatic activity enabling the digestion of carbohydrates normally appears during the end of the second trimester of pregnancy. Enzymes responsible for the digestion of proteins and lipids are activated shortly thereafter. Another prerequisite for successful and independent feeding of a child after birth is the establishment of the functional gut motility.
In general, a baby’s digestive system is already similar to its adult counterpart. However, differences do exist due to immaturity of the organs, their function and cooperation. One of the most relevant differences is the small stomach volume of a newborn, which is only able to hold about 60-90 ml. This has logical implications for feeding frequency and tolerance. The immature gut motility (i.e. the ability to move gut content through its lumen) is another major notable difference. During later phases of life, impaired motility is also one of the main symptoms of functional gastrointestinal issues and gastrointestinal infections (Dumont and Rudolph, 1994).
After birth, the immature digestive system of a newborn needs to adapt to an environment very different from conditions in the uterus. This adaptation takes place during a transitory period and may lead to a loss of up to 10% of an infant’s birthweight. From a nutritional point of view, this transition represents the most important change a human being undergoes during his/her whole life. In order to successfully complete the process, a necessary precondition is the sufficient development of physiological functions in the alimentary tract. They allow the digestion and absorption of macromolecular nutrients. The onset of various biochemical processes – especially energy-providing processes, including brain metabolism – is a second prerequisite.
This development is facilitated by breastfeeding – just another example of the critical role that breast milk plays in the development of both newborns and premature infants (Neu, 2007). Agents available in breast milk that affect the growth, development and function of the neonatal gut epithelium, immune system or vegetative innervation of the gut, are summarised in the following table (Goldmann 2000). It is obvious that no single substance has an effect on all aspects of gastrointestinal physiology, yet the cocktail of bioactive factors in total affects all systems of an infant. This could be one of the developmental reasons for the rich contents of breast milk.back
The human gut contains a vast quantity of various bacteria constituting what is referred to as the microbiota. The microbiota is metabolically active and supports various physiological functions, especially immune defence. Although opinions differ, at this point in time the prevailing theory is that there are no live bacterial communities within a healthy foetal in utero environment (Perez-Munoz et al, 2017). Consequently, an infant’s gut microbiota starts to evolve only after birth and the transition to the final composition is coupled with the transition to family food. Therefore, the process generally ends during the child’s second year of life.
This progression is visually summarised in the following figure (Tanaka and Nakayama, 2017). As shown, many outside factors can disrupt the process of maturing the gut’s microbiota. If such interference prevents the proper completion of the process, it can have a significant influence on immune diseases such as food allergies or asthma (Stiemsma and Turvey, 2017).
Again, breast milk helps by providing several beneficial factors, supporting the colonisation of the gut with beneficial probiotic bacteria. Indeed, it seems that breast milk represents a universally positive source of nutrition, protection, and good health. Moreover, weaning plays a critical role because the introduction of solid foods leads to major shifts in microbiota composition. Childhood then stabilises the microbial community in line with dietary patterns. In contrast, heredity seems to play only a minor role.back
By the 24th weeks of gestation, the foetal intestine has developed morphologically and physiologically to such an extent that it is equipped for most functions necessary for extra-uterine activity. This includes hydrolytic and absorptive functions of gut epithelia, development of immune structures associated with the gut, and its endocrine function (Xu, 1996).
Figure 1 summarises the development of enzymatic activity of the small intestine’s brush border cells. From the nutritional point of view, it should be stressed that by the 24th or 25th week of gestation, the activity of enzymes which are essential in the digestion of important carbohydrates is fully present. This means that by the currently accepted limit of preterm viability, carbohydrates appearing in preterm formula such as lactose or various polysaccharides can already be digested. Details are shown in the two figures attached (Lentze, 2012).
Enzymatic activity of trypsin and chymotrypsin – enzymes responsible for the digestion of protein – develops fully during the 26th week of gestation. These enzymes are activated by enterokinase appearing in the brush border epithelium (see Figure 2). A similar process can be observed with various lipases which are active in the digestion of lipids (Armand, 2007).
In summary: On the one hand, the gut of a preterm baby is essentially prepared for oral nutrition from approximately the 25th to the 26th week of gestation. On the other hand, complications can still emerge with enteral feeding of preterm babies. The underlying problem is usually to do with gut motility; with gut peristalsis being unable to cope with bolus nutrition. Such full postprandial peristaltic activity appears only sometime after the 30th gestational week. This fact has to be taken into account when planning the nutrition of preterm infants.back